In general, a tool holder has a function of holding by clamping a tool such as a drill for mechanically processing a process object in a spindle unit of a machine tool.
In this conventional tool holder, as shown in FIG. 1 (a), as a taper collet-chuck method, a tool (T) is inserted to a taper collet 10 and a force is applied in an axial direction by a collet nut 12 to provide a component of force in a radial direction at a taper surface to clamp the inserted tool. As shown in FIG. 1 (b) and FIG. 1 (c), as a hydraulic-chuck method, a hydraulic chamber is formed inside a tool holder to elastically change an inner surface of a tool insertion inlet through a hydraulic force and to clamp the inserted tool, while as shown in FIG. 1 (d), as a shrink-fit method, high temperature heat is applied to heat-expand a tool mounting unit such that the tool is clamped by stress generated by interference of the tool mounting unit that is restored to an original shape at room temperature and a tool shank unit.
However, the conventional art has the following problems.
That is, the tool holder applied with the taper collet-chuck must be installed with a separate device for applying the force in the axial direction such as a spring or the collet nut as the taper collet to the spindle unit such that the entire structure of the tool holder is complicated such that an accurate process and assembly technique are required, and a shape error of the taper collet is accumulated, and as a result, a problem that a geometrical tool set-up error is increased is caused.
Also, deformation by centrifugal force decreases a taper contact ratio under high rotational speed of the tool holder and the tool position error of the axial direction may not only be generated, but also the structure of the spindle unit is further complicated because of adding a draw bar and a press control device in the axial direction inside the spindle unit to realize an auto tool exchange device such that a vibration mode characteristic of the spindle unit may be damaged.
Secondly, in the tool holder applied with the hydraulic-chuck method, when clamping the tool by using the high pressure, as shown in FIG. 1 (b), the separate pressing system is required such that the entire system is complicated and an auxiliary cost is added.
Also, in the case using the low pressure, as shown in FIG. 1 (c), a pressure bolt 14 is rotated at one side of the tool holder to increase the pressure, and in this case, it is difficult to realize the auto tool exchange device, and the position of the pressure volt and the arrangement of the hydraulic line inside the holder do not form axis symmetry such that a structural mass imbalance factor is generated, and as a result, the rotation accuracy of the high rotational speed is decreased.
Thirdly, the tool holder applied with the shrink-fit method applies the high temperature heat to exchange the tool such that the tool holder is moved to an additional heating system for the tool exchange after the tool holder is separated from the spindle unit, and thereby the entire system is complicated and the auxiliary cost is increased.
Also, for the unclamping of the tool, when heating to the high temperature, 1-2 minutes are taken if necessary such that the time for the tool exchange is increased, and as a result, work productivity is deteriorated.